HEARING AID TECHNOLOGY (CIRCUITRY)

Until recently, the goal of hearing aids was simple: to make all sounds louder. Today, the goals also include making soft sounds sufficiently loud while preventing others from becoming uncomfortably loud and emphasizing sounds that are meaningful while deemphasizing those that are not.

As mentioned, hearing aid style refers to how a hearing aid looks and how it’s worn in or on the ear. Hearing aid circuitry refers to the technology inside. At present, existing hearing aids use three types of technology: conventional analog, programmable analog, and digital. With a few exceptions, any type of circuitry can be placed in any hearing aid style. In other words, style has nothing to do with the sophistication of the circuitry inside. The type of technology determines the extent to which a hearing aid can selectively process incoming sounds and the degree to which the aid can be customized to meet individual needs.

Conventional Analog Technology

Until recently, all hearing aids used analog technology to process incoming sounds. For the most part, analog hearing aids simply make all sounds louder. The hearing aid’s microphone collects sound waves traveling through air and changes them into electrical signals. The electronic copy of an incoming sound—the sound’s “analog”—is amplified (made bigger) and sent to the hearing aid’s receiver, which changes it back into sound. The signal that a listener hears is louder than the original (with some distortion thrown in). It may be possible to emphasize some frequencies (for example, the high frequencies) over others; however, this type of selective amplification is crude. The audiologist makes all adjustments manually (on the hearing aid) using a tiny screwdriver. Conventional analog hearing aids are the least flexible in terms of meeting individual needs. Available features generally are limited to telecoils and manual volume controls (see later discussion). However, they are also the least expensive. Although this technology is becoming obsolete, it might be perfectly adequate for people who usually communicate in quiet situations (like people who live alone and don’t go out often). It’s less effective in difficult listening environments because signal processing features can’t be integrated into the circuitry.

Programmable Analog Technology

With a computer microchip inside, programmable analog hearing aids are a step up from conventional analog hearing aids. They still rely on analog circuitry to process incoming sounds, but digital technology (a microchip) is used to program them. An audiologist attaches the hearing aid to a personal computer and uses special software to program the aid for the listener’s hearing loss. This is quite an improvement over using a screwdriver to make crude adjustments on an analog hearing aid. The hearing

signals are amplified, and then converted back into sounds by the hearing aid’s receiver. In a digital hearing aid, however, an analog-to-digital (A/D) converter changes the electrical signals into a computer code (millions of zeros and ones). The computer then applies complicated algorithms (mathematical formulas) to the incoming streams of numbers to analyze sounds in the environment (are they noise, speech, music, feedback?) and decide how they should be processed. Based on those decisions, different features perform complex actions on the signal. Millions of adjustments are made, theoretically providing the clearest, most natural sound quality possible. Reducing sounds to numbers allows them to be processed in an infinite number of ways, very quickly, and without distortion. After adjustments have been made, the streams of numbers are converted back into electrical signals by a digital-to-analog (D/A) converter. The hearing aid’s receiver then changes the electrical signals back into sounds that get sent into the ear canal. And all of this happens in an instant!

HEARING AID FEATURES: DIGITAL SIGNAL PROCESSING

Different hearing aid models incorporate different features or combinations of features. These features have been designed to solve the problems long associated with hearing aid use (for example, understanding speech in noisy situations, acoustic feedback, and uncomfortable loudness). Conventional analog aids offer virtually no signal processing features, and programmable analog aids offer very few. Larger hearing aids (BTEs and ITEs) can accommodate more features than can smaller aids (ITCs and CICs). And, of course, more features cost more money.

In many cases, digital signal processing means greater listening comfort and improved speech understanding in difficult listening conditions. However, no hearing aid—no matter how advanced—can block out all background noise, nor can any hearing aid eliminate the distortion that can be caused by sensorineural hearing loss. Having realistic expectations increases your probability of success with hearing aids.

The research and development teams employed by hearing aid manufacturers continually search for new ways to solve old problems. The strategies that manufacturers develop are often proprietary (patented) and called by different names. This makes comparing hearing aids like comparing apples and oranges (and dozens of other fruits). Manufacturers routinely claim that their particular strategy or feature is “the first,” “the only,” or “the best” solution to a particular problem. Your audiologist can help you to sort through advertising claims and set priorities.

Some examples of the features available in hearing aids today are included here. The intent is not to showcase any particular processing strategy or any particular manufacturer’s features; rather, the intent is simply to give you a sense of what’s available. Dozens of features exist, and they are continuously evolving. By the time you read this, new ones will have been

developed. These examples, described in simplistic terms, are meant to be representative only.

Adaptive and Automatic Sound Processing

Digital technology made it possible for hearing aids to offer sophisticated signal processing features, but the features were sometimes cumbersome to operate. Sometimes it was difficult for users to know when to use advanced features. Other times, it was difficult for users to make adjustments in ever-changing listening environments. And as more processing features became available, adjustments to one feature had the potential to interact with adjustments to another. It seemed that listeners might not be able to take advantage of the features they had. Today, intelligent or smart hearing aids have features that work adaptively and automatically. The hearing aid continually analyzes the listening environment by performing millions of calculations (per second), and then adapts to it automatically. The hearing aid makes internal adjustments so that the user doesn’t have to make manual adjustments; the aid is essentially on “auto-pilot.” In contrast, when aids are not automatic, the user considers the listening environment and makes adjustments manually by touching a button on the hearing aid or using a remote control (for example, the listener can adjust the volume manually or change listening programs to suit the environment). Many listeners appreciate never having to adjust their hearing aids, but others prefer to be able to make adjustments themselves. Some hearing aids are fully adaptive and automatic, yet allow the user to manually override some of the computer’s decisions by using controls on the hearing aid or a remote.

Manufacturers of fully adaptive and automatic hearing aids claim that they provide the best combination of sound quality, speech understanding, and comfort in all listening situations. The computer finds the optimal combination of settings and provides the best amplification scheme for any environment. The good news: all this happens in a heartbeat and is imperceptible to the listener. The bad news: there are still situations in which listening is difficult.

Data Logging

Some hearing aids analyze and store information about the amount of time a listener spends in different listening environments and how the hearing aid’s features are adjusted (manually or automatically) to accommodate those environments. That is, the hearing aid keeps track of its own performance and builds a profile of the listener’s needs and preferences. The audiologist can access the stored information and use it (along with the listener’s feedback) to further fine tune the hearing aid’s features. In some cases, the hearing aid’s computer can even make suggestions about how settings can be improved.

Some hearing aids are trainable. This technology is based on the observation that individual listeners have consistent preferences about how they like to hear in different environments. For example, as a listener adjusts the volume control to his preferred setting in a variety of listening environments, the trainable hearing aid remembers and learns. Soon, the hearing aid automatically adjusts volume to levels that match that listener’s preferences. A possible future development is an experience monitor that periodically signals the listener (through the hearing aid) that a test is about to take place. A paired comparison of two different feature settings would be presented, and the listener would “vote” her preference by pressing a button on the hearing aid. Over time, the hearing aid would learn the listener’s preferences in various environments, and the monitor would become unnecessary.

Multichannel Processing

Most digital hearing aids (entry level to premium) divide incoming sounds into different frequency “channels.” In the very simplest example, there would be a high-frequency channel and a low-frequency channel. Sound processing is controlled independently in each channel; for example, high-frequency sounds might be amplified more than low-frequency sounds. The way in which signal processing features are programmed in one frequency region (channel) doesn’t affect the way they’re programmed in other regions. Theoretically, processing sounds in smaller “slices” is more precise; the greater the number of channels, the greater the ability to tailor a hearing aid’s performance to meet an individual listener’s needs.

Loudness Compression

Listeners with sensorineural hearing loss (more specifically, listeners with damage to the cochlea) are likely to find loud sounds uncomfortable. In fact, “too much loudness” has long been a complaint among hearing aid users. In an effort to solve this problem, several loudness compression strategies have been developed. All of them share the same basic goals: to ensure that soft sounds are amplified enough for the listener to hear them but to prevent other sounds from becoming uncomfortably loud. In other words, the normal range of loudness is compressed to fit within the listener’s abnormally small comfort range (also called his dynamic range; see Chapter 4). This feature is now basic to most digital hearing aids, including those in the entry-level category.

Without compression, all sounds (whether they’re soft, medium, or loud) are amplified by the same amount until the hearing aid’s output limits are reached, and the sounds become distorted (as well as unpleasantly loud). With compression, softer sounds receive more amplification (but are still perceived as relatively softer), and louder sounds receive less amplification (but are still perceived as relatively louder). In hearing aids with multiple

channels (see previous discussion), compression settings can be adjusted independently in different frequency regions. This allows more compression in some frequency channels (perhaps the high-frequency channels where hearing loss and the need for compression are greatest) with less in others.

Digital Feedback Reduction

Feedback is the annoying (and often embarrassing) high-pitched squeal that a hearing aid sometimes makes. It happens when sound that’s been amplified leaks out of the ear canal and passes through the hearing aid again. It’s more likely to happen when sound is trapped close to the ear— as when a listener cups her hand around the hearing aid, wears a hat over the hearing aid, gets a hug, or uses the phone. Feedback also occurs when the earmold or hearing aid shell is poorly fitted or inserted into the ear improperly, or when the aid’s volume is turned up too high. Occasionally, it means the hearing aid is malfunctioning.

Before the availability of digital signal processing, feedback was controlled by turning down the volume on the hearing aid. This reduced the feedback, but it also reduced the benefit provided by the hearing aid. With digital signal processing, most (but not all) feedback can be controlled by the hearing aid’s computer. When feedback occurs, the hearing aid detects it and introduces an internal signal that’s exactly the same but 180 degrees out of phase. Signals that are the same but opposite in phase cancel one another. Digital feedback management has made the new generation of open canal fittings possible. Without it, sound would leak out of the open ear canal and cause constant (or nearly constant) feedback. This is another basic feature now found in most digital hearing aids (entry level to premium).

Directional Microphones

Difficulty hearing in noisy conditions is the most common complaint among hearing aid users. The directional microphone is a feature that’s been shown to improve speech understanding in noisy environments. In fact, this is one of the two best technologies available today for improving speech understanding in noise (read on for the other one). Traditionally, hearing aids have had one omnidirectional microphone that picks up sounds from all directions. Omnidirectional microphones most closely represent the way we typically hear—from all around us. They give listeners a sense of where sounds are coming from and the most natural connection to the rest of the world. Today, most digital hearing aids (entry level to premium) also have a second microphone, called a directional microphone. In the simplest example, a directional microphone emphasizes sounds coming from one direction (usually the front), while deemphasizing sounds coming from other directions. This means that in a noisy situation, the directional

microphone targets conversation (which is usually in front of the listener) and reduces (but does not eliminate) distracting sounds coming from the sides and rear. This works best when the user is close to the sound that she wishes to hear and the noise is coming from another direction.

Depending on the hearing aid, switching between microphones may be done manually (by pressing a button on the hearing aid or a remote control) or automatically (the hearing aid’s computer analyzes the listening environment and selects the most appropriate microphone arrangement). Directional microphones work best with BTEs; they’re also available in ITEs and, less often, in ITCs. Directional microphone technology isn’t possible with CIC hearing aids because of their small size and position deep within the ear canal.

Digital Noise Reduction

Many digital hearing aids (entry level to premium) have sophisticated features designed to reduce noise and enhance speech. Each manufacturer has developed its own approach. In general, a hearing aid with a noise reduction feature performs millions of calculations (per second) to determine whether incoming sounds are speech or noise (or some combination of the two). In a simple example, the hearing aid recognizes speech sounds as short and constantly changing and noise as steady and continuous (for example, fan noise, appliance noise, or traffic noise). In this case, the solution is to reduce amplification in the channels containing continuous sounds while preserving the amplification of speech sounds in all other channels.

Noise reduction systems do a reasonably good job of analyzing the sounds coming into a hearing aid; that is, determining whether they’re speech or noise. They’re not as good at treating the two signals differently. The speech signal is made up of a broad range of frequencies; therefore every frequency channel contains some of the speech signal. When amplification is reduced in a channel containing noise, the speech sounds in that channel are reduced as well, and that makes speech harder to understand.

Hearing aids amplify all sounds, not just those we want to hear. To some extent, a person’s success with hearing aids depends on his ability to tolerate background noise. Many hearing aids are capable of emphasizing some sounds over others, but no hearing aid can completely separate unwanted sounds from those that are of interest. Clinical tests generally fail to show an improvement in speech understanding as a result of digital noise reduction. Nonetheless, users often report that noise reduction makes listening in noisy environments more comfortable, less stressful, and less tiring. And many users think that’s worthwhile.

Wind Noise Reduction

This is a special type of noise reduction. Some hearing aids can be programmed to recognize wind noise and suppress it, while preserving

the speech signal to the extent possible. In extremely windy conditions, some hearing aids can momentarily reduce all amplification to prevent discomfort.

Multiple Listening Programs

Most digital hearing aids (entry level to premium) can store several preset programs in their memories. The audiologist creates different programs for different listening situations—it’s like having several different hearing aids all in one. For example, a listener might have different programs for listening in quiet, in noise, or on the telephone. Another listener might have a program for comfortable listening and a different program for critical listening when it’s important to be able to hear very well for a brief period of time. Yet another listener might have a program for bird watching. Some listeners have a program especially designed for appreciating music. The listener can change programs by pressing a button on the hearing aid or using a remote control. Many digital hearing aids change programs automatically. Without multiple programs in memory, the same internal hearing aid settings are used for all listening environments.

My Dad

The right hearing aid in my dad’s BICROS hearing aid arrangement (see earlier description) had three preset programs. Program 1 (signaled by one beep) was used for quiet conditions; it used the omnidirectional microphones on the right and left ears. Program 2 (signaled by two beeps) was used for noisy conditions; it used the directional microphone on the right hearing aid and the omnidirectional microphone on the left ear. Program 3 (signaled by three beeps) was also used for noisy conditions; it used the directional microphone on the right hearing aid and deactivated the microphone on the left ear. Deactivating the microphone on the left side reduced noise coming from anywhere but the front (where the directional mic on the right ear was pointed).

My dad changed programs manually by pressing a button on his right hearing aid. My dad’s programs were simple. In many cases, switching among programs means more than switching microphones on and off.

My dad’s new hearing aid is fully adaptive and automatic (that is, it analyzes the listening environment and then automatically makes adjustments that adapt to it). However, I also wanted him to have a program switch that would allow him to override the automatic adjustments. Program 1 is for quiet listening conditions (omnidirectional microphone), Program 2 is for noisy conditions (directional microphone), and Program 3 is for critical listening situations (in which he needs extra volume). He also has a program for the telecoil setting (see later discussion and Chapter 9).